Biosensors have been and are being developed to detect, identify and quantify various biochemicals, ranging from proteins to toxins to RNA to c-DNA to oligos and to disease agents such as viruses, bacteria, spores and Prions. This list is by way of example, and is not intended to be complete. Some biosensors sense charge on the molecule. Many biochemicals carry a net charge. Electrophoresis methods and various blots exploit molecule net charge to affect physical separation of such molecules.
There is a significant problem with existing techniques such as electrophoresis and the various blots. These sensors are not specific in identifying the molecules in question unless significant post processing and labeling is employed. Further, a very large quantity of the tested biochemical is required for electrophoresis detection methodologies.
In many instances the number of molecules available for detection is very small and may be below the sensitivity threshold of the sensor, or may be problematic with respect to sensitivity. For example, some plasma proteins are of very low concentration. Toxins such as Botulinum toxin are notoriously hard to detect at lethal thresholds because of their very low lethal and sub-lethal, but still dangerous, concentrations. Mass spectroscopy requires a large number of molecules in order to achieve adequate detection sensitivity.
In the case of c-DNA and RNA sensing, the number of base pairs or nucleotides present may be low creating a problem for adequate detection and identification. This is possible if, for example, only a few bacteria are present or the RNA is of low concentration because of either the desired identifying component or small virus count. Virus RNA may be of low density for samples monitoring air. Only a small portion of the RNA may provide the definitive identification signature. Overall these sensing environmental aspects may lead to a relatively small amount of RNA or DNA actually involved in the definitive detection process.
In the case of proteins, the target molecule concentration may be very low in the sample. For example, with Prions (mad cow disease), if a fluid sample is taken from an animal's blood, the target protein concentration can be very low compared to that taken from brain tissue. With early infection of humans, animals or plants with disease, the initial signature indicators may be present in only very small concentrations. Signature bio compounds for a liver infection at its onset or signature proteins in the very early stages of cancer, when one wishes to identify disease presence, are typically of very low concentration. Key definitive disease indicators may be present in only very small concentrations.
Where only small concentrations of target molecules are available, mass action effects can result in the bound target concentration being very low. A small percentage of the actual receptors, e.g., specific antibodies to the target molecule, available for bonding of the target molecules can result in a very small detection signal. For example, consider the case of a lethal concentration of botulinum toxin. Only a very tiny fraction of the receptors (recognition elements) are bound, perhaps 1 in 3000 antibodies. At the very earliest onset of disease, the density of indicative proteins, viruses, antibodies and bacteria may be very low, thus placing a very high sensitivity measurement burden on the sensing approach.
Sensors for the detection of target molecules using charge have been reported. The most commonly used to date are those using electrophoreses methods, such as the various blots. Semiconductor charge sensors have long been highly sought after due to their compatibility with integrated circuits and attendant low cost batch manufacturing processes. An example is the ImmunoFET that uses a conventional MOSFET, absent a metal gate, and employing a reference electrode in solution. This approach is problematic resulting in drift and small signals.
Biochemicals such as proteins and receptors often carrier charge. By way of example, blots and electrophoresis exhibit details of such biochemical incorporated net charge. The amount of charge on the biochemical is often pH dependent.
When measuring attached charge arising from biochemicals bound to a biosensor, the link to the density of molecules attached incorporates knowledge of how much charge is bound by each molecule. Such bound bimolecular charge is well known to depend on pH.
Some measurements are usefully performed at different or variable pH values. Indeed, the charge dependence on pH can provide some confirmation that the biomolecule attaching is the one expected or targeted.
Semiconductor devices are often temperature dependent. While temperature-correcting circuits may be integrated and invoked to correct the measurement output for temperature influences, an alternative method is to measure the temperature and correct the sensor output, e.g., using a look up table.
Temperature can be used to affect molecular dissociation as is commonly done with DNA. The temperature at which such dissociation occurs is a measure of nucleotide binding and molecule details. Temperature measurement under these conditions can provide useful quantitative information related to the biochemical.
While external means may be employed to change the sample environment's temperature, it is more convenient at times to control the temperature change on chip automatically. This is done using a heater feedback systems linked to a temperature monitor.
PCR incorporates thermal cycling. Incorporation of PCR with selective sensors can provide utility. For example, the thermal cycling may be combined with suitable wash steps and reinsertion into the reaction region to collect target molecules provides a means for collection of targeted molecules. By way of example, such target molecules may be RNA molecules or antibodies. Other examples are possible.
There is a need in medical science for reliable, low costs medical diagnostics, as well as reliable and robust universal biochemical sensors for disease diagnostics.
Needs exist for biochemical sensors that provide an electrical output sensing parameter that is easily measured. The biosensors need to be suitable for automated testing, e.g. to be employed in medical laboratories that manage large numbers of diagnostics samples. Biochemical sensors must be able to interrogate a sample for any one of many targets or diseases simultaneously.
Improved biosensors need to be compatible with electronics in general and with integrated circuit technology in particular. Such biosensors would easily access a wealth of known integrated circuit (IC) technologies and integrated circuit electronic means of measurement and interrogation.
There is a need for biochemical sensors with high sensitivity (for low target concentrations and multiple target diagnostics) and which can be easily measured to provide details of biochemical detection details.
Ease of measurement is an important desirable biosensor feature. Ideally a simple multimeter and, if needed, a simple power supply are used. To address these, the sensor designs must be such as to provide ease of measurement.
There is a need for low cost biosensors. Batch processing technology offers advanced low cost manufacturing processes for fabrication of many types of electronic devices and circuits. IC batch processing can be used to manufacture very low cost biosensors. IC technologies are particularly attractive since these are readily available in IC foundries. Such technologies include, but are not limited to, polysilicon deposition, doping (including ion implantation), surface state control processes, patterning and photolithography. In the best manufacturing case, such technologies best do not push to nanometer dimensions due to costs, low yields and other adverse features. It is desirable to ensure consistent device performance and sensor arrays (hybrid or integrated on chip) with all or the vast majority of sensors working, all compatible with convenient measurement range using readily available instrumentation or circuit design. Excessive need for signal processing adds costs.
Needs exist for biosensors that are not prone to 60, 120 Hertz, microphonics or other interfering pickup from stay interfering signals in the environment.
There is a need for sensor arrays that can be used to simultaneously tests for multiple targets in an environment. Such arrays should be incorporate sensing components that are easily measured using conventional circuitry and/or instrumentation and not push signal processing limitations.
Biosensor arrays with sensor spacing compatible with robotic liquid handling (spotting) are needed to provide multiple target recognition element placement without bleeding of spots. Bleeding of spots can undesirably corrupt specific recognition objectives of adjacent sensors in the array.
There is a need for automated circuit support of biochemical sensor arrays. Semiconductor technologies, especially silicon IC technologies, can provide integrated circuit needed function in circuit form on the same chip as one or more sensors.
Needs exist for sensors and sensor arrays compatible with simple packaging schemes.
Sensors should be compatible with both advanced IC technologies and advanced biochemical technologies. Biochemical technologies can be integrated with Si technology to incorporate a wide range of recognition element suitable for targets such as those listed in Table 7.
There is a need for biosensors that are insensitive to moisture and fluids found in the testing environments. Typically, semiconductor based devices show a significant moisture sensitivity leading to erroneous measurements. Sensing for targets in a liquid such as water or blood plasma is a requisite for biochemical sensors. Water, plasma or other target containing liquids should not significantly influence the sensor. The same is true for the attachment of recognition elements incorporating a liquid carrier. Whereas there are methods to provide water protection for Si integrated circuits, these methods necessarily often introduce undesirable features that are undesirable, for example, emersion in a binding epoxy resin. In particular, very thin insulating over layers are suitable for measuring contact potential of a coating or partial coating material. However, such thin over layers typically do not well protect against water contamination.
There is a need to provide overlaying insulators which are thin to ensure excessive stress is not developed, such as can occur for very thick Si3N4 layers.
Needs exist for sensitive biosensors that have low cost measurement instrumentation requirements. To be widely employed, the attendant biosensor instrumentation needs to be low cost.
There is a need to provide sensitive biosensors which use measurement instrumentation for which health workers can easily be trained, and/or which equivalent instrumentation can be provided on chip in support of simple diagnostic readout devices.
Biosensors are needed for being employed successfully, without loss of utility, where inhomogeneous target or recognition element attachment has occurred. That is, inhomogeneous influences on the sensor conducting regions caused by inhomogeneous recognition element and/or target attachment should not be problem in introducing errors in either detection or quantification.
There is a need for biosensors that use relatively simple circuitry to provide an instrumentation or measurement function, on chip.
Easy remote sensing of biosensors is needed. Such addresses needs for homeland defense. Remote sensing further supports disease diagnostics, for example in remote regions where unskilled workers may be attending the ill, which may, by way of example, be important in tracking the spread of an epidemic or pandemic.
There is a need for biosensors that enable hybrid or integrated sensor arrays that are easily integrated into communications and automated measurement systems.
Needs exist for semiconductor biosensors that are compatible with standard integrated circuit technologies, such as CMOS technology.
Improved pathogen detection sensitivity, which can be affected using compound semiconductor high mobility materials, is needed.
There is a need for very high sensitivity biosensors that can detect multiple present diseases with a single low cost device.
Needs exist for high sensitivity simple biosensors that can incorporate only a few expensive recognition elements where said recognition elements are very expensive. The more sensitive the device, generally the fewer recognition elements are needed. Larger dimension devices are design for sensitivity and can detect many different targets and keep the recognition element costs low. For example, reducing the number of antibodies needed for detection by a factor of 100 reduces antibody costs by a factor of 100. Spread over millions of biosensors, this cost saving is substantial.
There is a need for chemical sensors that can provide a measurement of the Gibbs free energy of a material, i.e., measure the Fermi level of the material. For example, such devices can be used to monitor environmental degradation of materials.
Avoiding unwanted surface trapping states is desirable when it would diminish or eliminate the electric field influences on the conducting channel free carriers. There is a need to avoid charge screening of the target charge.
Needs exist to provide a large enough signal such that the influence of the target attachment is easily measured with relative simple instrumentation or circuits, such as an ohmmeter. This need relates to resolution of multiple binding of targets and the ability to make reliable measurements at low cost.
Sufficiently large area devices are needed if robotic spotters are to be used to apply recognition elements at different locations, either on the same bioresistor or on arrays of bioresistors. If the bioresistor's surface is sufficiently large, high sensitivity designs permit multiple target sensing on a single sensors. There is a need for sensors to be of sufficient dimension such that robotic spotters can apply local attachment of pre-selected recognition elements. While such robotic systems can apply multiple chemical mixes to different location on a single chip, the minimum size of the liquid spot is limited. Bleeding from one sensor to an adjacent sensor can occur if the sensors are sufficiently small and densely arrayed. Such cross mixing is adverse and can introduce errors of measurement and false positives or false negatives. For example, substantially sub micron or nano dimensional electrical sensors are generally undesirable for these and other reasons.
There is a need for the biosensor active area to be sufficiently large to define pH controlling mixed functional groups in order to help ensure a constant pH at the sensor surface and avoid unwanted and unknown localized pH fluctuations which may affect the charge on the target, as is known to be a function of pH.
Needs exist for highly sensitive biosensors that can sense very low concentrations.